Pressurised water nuclear reactors have a primary circuit for circulation of the cooling water of the reactor and a pressuriser disposed on a branch of the primary circuit in order to maintain the water pressure in the primary circuit between certain predetermined limits. The pressuriser also carries out the function of compensating for variations in the volume of water in the primary circuit during the operation of the reactor.
The pressuriser comprises a water tank, elongate heating elements or “heating rods”, extending in a lower portion of the tank, and a spray nozzle for spraying cold water in the upper portion of the tank. The pressuriser contains water in its lower portion connected to the primary circuit and a vapour bubble in its upper portion. Controlling of the water pressure is done by evaporating the water by means of heating rods and then condensing the vapour by spraying so as to control the volume and pressure of the vapour bubble.
The tank is elongated along a longitudinal axis, oriented vertically. The heating rods pass through a bottom wall of the tank and extend into the interior of the tank parallel to the longitudinal axis.
The pressuriser includes at least one spacer plate, generally two spacer plates in order to hold the heating rods in place transversely. Each spacer plate is fixed in the interior of the tank.
While in operation, a heating rod could become faulty, such that it becomes necessary to proceed with its extraction and possibly its replacement.
In order to extract a heating rod, it is normally required that a downwards traction (pull) be exerted on the heating rod to extract it through the bottom wall.
However, there are cases in which the defective heating rod has a deformed section making it impossible to perform its extraction through the bottom wall of the tank by exerting downwards traction.
Document FR 2666679 discloses a method for the extraction of a defective heating rod having a bulge, wherein the heating rod is cut and then the sections of the heating rod are extracted through an inspection—service opening provided in a top wall of the tank.
However, this method has several drawbacks which make the implementation thereof long, difficult and risky.
On account of the geometry of the spacer plates, accessibility of robotic devices and their handling with no direct visibility is very difficult, which is similarly true for the extraction of the heating rod sections. The existing risks include the generation of debris that would be difficult to recover, as well as loss of the robot and damage to adjacent rods. Furthermore, this process does not provide for the restoration of the surface condition of the openings for passage of the heating rods in the spacer plate or plates, which prevents the subsequent replacement of the damaged heating rods by new rods.